I'm going to give you an answer from my several decades of servicing Brit bikes, which by the way have absolutely terrible mechanical advance systems. Others more knowledgeable to BMWs may chime in, but here goes...

You'll want to strobe time the engine at full advance, because....
1) The strobe will enable you to literally see how your AAU is working/ not working. (e.g. weak springs will show up as very fast advance.)
2) If you are ever going to damage the engine via ignition timing, then it will always be at highway speeds. [i.e. full advance]
3) You may over-heat the engine after a long idle, but timing at idle is not 1/10th as critical to engine health as full advance. Ignition timing only has to be in the "ballpark" to achieve good starting and idle characteristics.

The spark boost and flywheel to me don't figure into the timing spec. The "wrench in the works" is the worn mechanical advance, which is fully accounted for by timing at full advance.

You may not presently own a strobe, but you will never regret buying one to get this setting correct. Secondly, should the AAU be revealed to be ready for the trash bin as indicated by the strobe, you absolutely MUST have a strobe to install an Electronic Ignition. So one way or the other, you will soon own a strobe timing light !!

🙂

Thanks W,
I do have a strobe light. I was shooting for setting the F mark, kind of ignoring the S mark during this. I understand your point and was already on board with this approach.
It's really about the drifting of the F mark that confused me....

A clarification on exact method here......
With the timing light in my right hand, my left hand reaching over the bike to the throttle to get it to 1800-2000 rpm, while hunched over peering into a small hole; I have a hard time holding the rpm steady. I initially see the F mark right in the window, but then it continues to creep "UP" past the window. I may be increasing RPM's here, but feel like I need another set of hands and eyes.
Is there an easier way to do this without a second person?

Specific question
1. Does the RPM need to be steady for the F mark to remain steady, or is "drift" a symptom by itself of advance unit wear or ??

Much appreciated as always,

Thanks for the info about higher RPM's. I'll try that to see if I have more stability with F position.
As to the advance unit, it is worn, but the springs are new. I have had it apart and cleaned up as much as I can. The weights move easily and pull back. The shaft part of the advance unit that hits the felt block is quite lumpy and scarred. I don't mean the bilateral higher aspects that open the points, the entire shaft is not smooth and clean metal. Maybe irrelevant to my timing issue, not sure of the tolerance on "ATU shaft smoothness".

I am thinking the "Alpha" electronic ignition system that replaces the mechanical advance unit with a rotor is the next step for better consistency in running. While ATU's are available if you look hard, they are almost as much as the alpha system.

I'll update my rpm/timing success tomorrow...

As Scot alluded to, forget all about the tachometer. Don't even look at it. Have a friend slowly rev the engine up from 1000 to 5000 RPM and set the timing so that the F mark appears in the timing window and stops there. It should NOT come into the window and then continue to climb until it disappears.

No matter what type, brand, model, power, year, etc, etc ignition system you install, the F should creep into view within the timing window and STOP there, no matter how high the RPM climbs. That is the very definition of the word "Full" within the term Full Advance.

► Now, due to the age and mechanical nature of a points type ignition, your F mark is not going to be rock solid. You'll see it dance around quite a bit. That's just simply the nature of mechanical advance units. But again, even with all the dancing, it should never leave the window once it climbs there, no matter how high the RPM climbs.

► Pitting on the ignition cam surface is inconsequential, as long as the cam surface is free of rust or other surface roughness that would turn the cam into a rotary file and prematurely wear the nylon cam follower on the points, thereby reducing point gap as you ride. Smooth it up with 400 grit and add ignition cam grease.

The part that makes the largest difference is the shaft the ignition cam rides on. That shaft-to-ignition cam clearance 1) needs lubrication to promote free rotation, and 2) needs to fit tight enough so that the points gap doesn't vary with RPM. A very loose fit will create erratic point openings, thereby creating random ignition sparks that you'll detect by using a strobe lamp.

Hope this helps.

Great info Wobbly, thanks a million....
My projects caught up with me, so I haven't gotten back to this timing chore. I will later with this info. I am seriously considering installing an electronic ignition system, as the ATU will keep wearing and eventfully cause more issues. If I'm putting 250$ into the bike, it will be electronic ignition, not another mechanical advance unit.

Bob

Bob -
You're so right. "Cost" has been the major reason people install electronic ignitions over the last 30 years. People rarely spend that type money doing "preventative maintenance" for the advantages. Most are faced, as you are, with spending a large chunk to merely restore an antiquated system, OR spend an equally large chunk to replace and start all new.

You'll enjoy the great idle and maintenance-free aspects of EI.

All the best.

Bike is on a lift at the shop of the local guru in my town. He is assisting me with installing EI, as well as replacing the diode board and rotor. (Thanks Kurt)
Dropped the exhaust system to sort out my dodgy centerstand, which is now fixed! That lean to the right with both tires on the ground didn't give me warm fuzzies. IE alomst installed, just need to finish connections and put it all back together, then the timing. Looking forward to see how timing may be different.
"Always something" has new meaning to me. Yea, it is a 40 year old bike.